1. Field of the Invention
The present invention relates to an apparatus, a method, and a computer program product for reading information stored in a storage medium.
2. Description of the Related Art
Recently there is widely known a semiconductor memory device such as a flash memory in which information is stored according to held charge amount. There has also been developed a multiple-value memory technology in which information on at least two bits is stored by setting plural thresholds of the charge amount.
In a charge accumulation type memory device, as time passes, and with each readout operation, discharge is performed due to a parasitic resistor of the memory device and an input resistor of a sense amplifier. Because a decrease in charge accumulation amount caused by natural discharge is generated even in the readout and writing operations are not performed for a long time, a state in which the information readout is disabled is easy to occur particularly when a non-volatile memory is not turned on for a long time. In the multiple-value memory, the discharge has a large influence on the decrease in output of the memory device, which causes the readout number to be restricted.
Therefore, for example, in a technology disclosed in U.S. Pat. No. 5,909,449, an output voltage of the memory device is detected before the state in which the information readout is disabled, and the output voltage is kept such that the information is correctly read by performing a refresh operation in which the information storage is re-started when the output voltage is lowered.
However, in the method disclosed in U.S. Pat. No. 5,909,449, when the output voltage of the memory device is largely lowered after a long time passes from the writing, there is a problem that the normal information for being written in the refresh operation cannot be read from a group of memory devices. Therefore, the refresh operation cannot be performed to the memory device in which a long time passes from the writing, which results in the problem that the information cannot normally be read.